SkyandTelescope.com August 2014 25the comet to do detailed mapping, select a landing site,
and deploy Philae safely. The activity level will increase
substantially as C-G approaches perihelion in August- At that point, the nucleus will be spewing out
about 4× 1026 (low-activity case) to 1× 1028 (high-activity
case) water molecules per second, which is about 30 to
80 gallons of water per second. For comparison, when
the instrument teams fi rst try to detect water and carbon
monoxide activity in July 2014 (when the spacecraft is still
more than 10,000 km from the comet), the production
rates are predicted to be a factor of 100 to 10,000 lower.
That is like trying to detect the evaporation of a thimble-
ful of water each second from somewhere on the other
side of Earth (assuming Earth wasn’t in the way).
The advantage of Rosetta is that we’ll be able to watch
and measure that activity over the course of the comet’s
orbit. The mission will spend an unprecedented amount
of time close to C-G, studying how its activity turns on,
how much of the surface is active or if that activity is pri-
marily from discrete jets, how the nucleus interacts with
the “collisional region” (the inner part of the coma where
the gas density is high enough for collisions between
atoms and molecules), and how all of that forms and
signal (though every minute felt like another year). It
was more than a decade from when the mission was fi rst
approved by ESA in 1993 to when it launched in 2004. The
craft has spent another 10 years in fl ight as we’ve waited for
it to reach its prime target, Comet C-G, this August. There
were long periods of waiting punctuated by episodes of
frenzied activity, such as when the launch was delayed due
to the failure of an Ariane rocket just a few months prior to
Rosetta’s launch date. The delay caused by the investigation
prevented Rosetta from launching in time to catch its origi-
nal target, Comet 46P/Wirtanen. The mission’s science and
operations teams had to scramble to fi nd a new target and
trajectory, plus pick diff erent asteroids for science fl ybys.
Pre-planning for a comet mission is tricky. The target
is constantly changing in unpredictable ways as its
distance from the Sun changes, so each moment in the
mission is unique. We don’t know ahead of time when
and where jets will sprout up on the nucleus, or even
where on the surface we’ll fi nd a spot for the lander. Nor
do we know for sure how strong the activity will be when
Rosetta arrives, so instrument and mission planners need
to prepare for multiple scenarios. All of this planning
also needs to be negotiated among the many instrument
teams, some of which have confl icting preferences, and
with the mission operations and fl ight dynamics teams
responsible for the fl ying and safety of the spacecraft.
We hope that C-G’s activity level is still low when
Rosetta arrives, so that the spacecraft can get close to
Surrounded by four 100-meter lightning towers, the fi rst Ariane
5G+ vehicle waits on the launch platform at the Guiana Space
Centre, Europe’s spaceport. Rosetta launched from here in 2004
and is now about to rendezvous with its comet.ESA / STEPHANE CORVAJA